A distributed antenna system (“DAS”) can include one or more head-end units and multiple remote antenna units coupled to each head-end unit. A DAS can be used to extend wireless coverage in an area. Head-end units can be connected to base stations. A head-end unit can receive downlink signals from the base station and distribute downlink signals in analog or digital format to a radio frequency distribution system, which can include one or more remote antenna units. The remote antenna units can transmit the downlink signals to user equipment devices within coverage areas serviced by the remote antenna units. In the uplink direction, signals from user equipment devices may be received by the remote antenna units. The remote antenna units can transmit the uplink signals received from user equipment devices to the head-end unit. The head-end unit can transmit uplink signals to the serving base stations.
Often, remote antenna units in a DAS can transmit and receive radio signals simultaneously in multiple frequency bands. Simultaneously transmitting and receiving signals, however, can cause signal distortion in the uplink paths of the remote antenna units. For example, downlink signals transmitted by a remote antenna unit or intermodulation products of downlink signals may leak into an uplink signal path of the remote antenna unit.
Remote antenna units can use a cavity filter for providing signal isolation between the transmit path and the receive path. Cavity filters, however, can be large and expensive, and can therefore be undesirable in remote antenna units. Other options that may allow a remote antenna unit to avoid using a cavity filter include reducing the uplink or downlink power. Reducing the uplink or downlink power, however, undesirably limits the upper power of the telecommunication system.
It is desirable to provide improved isolation between uplink and downlink path in a remote antenna unit.